The present invention concerns a change-speed gearbox for motor vehicles, and more particularly, to a change-speed gearbox having an input shaft, an output shaft, a countershaft located parallel to the input and output shafts, a respective drive connection between the input shaft and the countershaft via two gear stages arranged to be alternatively selected, and a drive connection between the countershaft and the output shaft via a further gear stage, in which a countershaft gearwheel of one of the gear stages has a smaller number of teeth than a main shaft gearwheel engaging therewith and only two of the three gear stages have the same total number of teeth.
A sixteen-speed change-speed gearbox for motor vehicles of the aforementioned type is shown in Publication F 43 239/RT 33 26-887, Page 10, FIG. 5, Zahnradfabrik Friedrichshafen AG, Lowentaler Str. 100, 7990 Friedrichshafen 1, Federal Republic of Germany. Three gear stages are used between the countershaft and output shaft to form one transmission ratio each for forward propulsion, and a fourth gear stage with an idler gearwheel is used to form a transmission ratio for rearward propulsion. Thus, by multiplication with the two transmission ratios of the gear stages used as constant mesh gears between the input shaft and the countershaft, eight transmission ratios are obtained for forward propulsion because the constant mesh gear adjacent to the main gearbox can also be employed as the fourth gear stage of the main gearbox, and two transmission ratios for reverse propulsion are also obtained. A two-stage range-change gear is also fitted downstream of the output shaft so that, with its two transmission ratios, there is a total of sixteen forward gears and four possible reverse gears.
In the known change-speed gearbox, the total number of teeth (i.e. the sum of the number of teeth of the countershaft gearwheel and the number of teeth of the main shaft gearwheel engaging with the countershaft gearwheel) of the two constant mesh gears is the same. In the gear stage between the countershaft and the output shaft which is used to form the third and fourth lowest transmission ratios between the input and output shafts, however, the countershaft gearwheel has a smaller number of teeth than the main shaft gearwheel which engages therewith. The known change-speed gearbox is, of course, designed for a certain permissible input torque with a magnitude of, for example, 1,600 Nm.
An object on which the present invention is based consists essentially in creating a range of gearboxes with several gearbox types for different permissible input torques while keeping the number of parts to a minimum.
The foregoing object has been achieved in accordance with the present invention by providing that a second countershaft gearwheel, associated with one of the two gear stages having the same total number of teeth, has a smaller number of teeth than an associated main shaft gearwheel.
In a change-speed gearbox according to the invention, it is now possible to start with a basic gearbox which is configured for a certain permissible input torque and has a gearbox construction in which a main gearbox operates with a plurality of gear stages between the countershaft and the output shaft. In one of these gear stages, the countershaft gearwheel is provided with a smaller number of teeth than the main shaft gearwheel which engages with it and which is seated, in this instance, as a loose wheel on the output shaft. Two gear stages used as so-called constant mesh gears are connected into the force path before the main gearbox. These gear stages used as constant mesh gears can be alternatively selected in order to produce a drive connection between the input shaft and the countershaft. These two gear stages have the same total number of teeth, i.e. the sum of the two numbers of teeth of one gear stage is equal to the sum of the two numbers of teeth of the other gear stage.
The present invention advantageously permits, as an initial possibility, a first gearbox type A of a gearbox range, in which gearbox the permissible input torque is increased relative to the basic gearbox and the slower of the two gear stages used as constant mesh gears in the basic gearbox is replaced by a gear stage in which the countershaft gearwheel has a smaller number of teeth than the main shaft gearwheel engaging with it which, in this instance, is seated on the input shaft. As a result, the constant mesh gear is now used to form the second highest transmission ratio of the main gearbox, i.e. that between the speed of the input shaft and the speed of the output shaft, and also for forming the lowest transmission ratio of the main gearbox.
The present invention makes a further gearbox type B possible in which the permissible input torque is increased relative to the gearbox type A and the countershaft gearwheel of the second gear stage used to produce a drive connection between the input shaft and the countershaft also has a smaller number of teeth than the main shaft gearwheel engaging therewith. The faster of the two gear stages operating as constant mesh gears is then again used to form both the second highest and the lowest transmission ratio of the main gearbox, i.e. between the speeds of the input shaft and the output shaft.
The gearwheel pairing of the "fast" constant mesh gear of the gearbox type A and the gearwheel pairing of the slower constant mesh gear of the gearbox type B can be identical.
The present invention also advantageously makes a third gearbox type C possible in which both the permissible input torque (relative to the gearbox type B) and the transmission ratio of the faster constant mesh gear (relative to the faster constant mesh gear of the gearbox type B) are increased.
The present invention further achieves the smallest possible number of parts because in the gearbox type B, the slower constant mesh gear is the same, in terms of transmission ratio and total number of teeth, as the fast constant mesh gear of the gearbox type A and the faster constant mesh gear can be the same, in terms of transmission ratio and total number of teeth, as the slower constant mesh gear of the gearbox type C and because the gear stage for producing a drive connection between the countershaft and the output shaft in the gearbox types B and C can be the same, in terms of transmission ratio and total number of teeth, as the slower constant mesh gear of the gearbox type A.
The present invention additionally makes possible in an advantageous manner a range with at least three gearbox types for each change-speed gearbox which has the otherwise basic known arrangement. Such arrangement can also be extended, in accordance with the present invention, to give a gearbox range irrespective of whether a range-change gear or downstream gearbox is additionally located in the force path downstream of the main gearbox which has the gear stages between the countershaft and the output shaft.
In terms of achieving the smallest possible number of parts, all the gearbox types associated with the gearbox range according to the invention have the same gearbox casing, the same gearbox shafts, the same selector clutches (synchronizing or claw clutches) and, with the exception of the two constant mesh gears and the one gear stage of the main gearbox, the same gearwheels also.
In the gearbox range according to the present invention, all the associated gearbox types have the same overall transmission ratio spread, the same gear steps and the same number of forward gears. Moreover, all the associated gearbox types can be operated by using the same shift scheme, provided a two-stage range-change gear is employed, using, for example, the known double-H shift pattern. The transmission range is displaced from gearbox type to gearbox type in the "fast" direction so that the higher permissible input torques also remain tolerable in the drive train after or downstream of the gearbox (e.g., for the propeller shaft, distributor gearbox, if present, and axles). The overall transmission ratio required (i.sub.gearbox .multidot.i.sub.rear axle) for the vehicle is achieved by the corresponding rear axle transmission ratio.